1. Field of the Invention
This invention relates to the field of optical waveguides, and more particularly, to a waveguide created by thermo-optically induced changes in the refractive index of a medium resultant from absorption of light focused thereupon.
2. Review of the Relevant Prior Art
The use of optical waveguides for directing the path of an optical beam has long been known in the prior art. In recent years, such waveguides have typically appeared in the form of "optical fibers" which may utilize the phenomenon of total internal reflection to confine light to a preselected path bounded by an interface between media with differing refractive indices. Aternatively, such fibers may employ a graded refractive index, referred to as a "grin" to direct light in a sinusoidal path down the length of the fiber, th graded index of refraction serving to bend the light to prevent it escaping the fiber. Such optical fibers have become increasingly important in such fields as communications since they provide a relatively low cost, easy to manufacture method of low energy, high speed transfer of information.
It is expected that devices employing optical energy, either alone or in combination with electrical energy, will come into increasing use. For example, while most conventional computers are actuated by electrical energy, it has become increasingly obvious that such electronic computers are limited in the speed at which information may be processed primarily due to the relatively slow path of the electrons through their conductive paths.
It has been proposed that components employing optical switching and transmission of information could partially or entirely replace the electronic components of such computers, with an anticipated improvement in operational speeds in a range of orders of magnitude. Unlike electrons traveling through a conductor, optical energy in a linear medium does not effect the optical energy in an adjacent medium, and, therefore, should enable greater packing densities of interconnects without cross-talk than can be achieved by electronics; furthermore the bandwidth of an optical signal is very high, enabling a given signal to carry large amounts of information.
It has long been observed that radiation passing through a non-linear optical medium can lead to a change in the refractive index of that medium. If the radiation is in the form of a focused Gaussian beam, the focal region where the radiation field is the strongest will have the greatest index change and the transverse spatial profile of this index change will be bell-shaped since the transverse radiation profile is also Gaussian. For a large F-number, focused beam impinged upon a nonlinear medium, wave mechanics dictate that the focal region is a long cylinder with a transverse graded index (GRIN) quite analogous to a graded index waveguide or fiber. Similarly, by focusing a beam having the appropriate (Bessel function) transverse field distribution onto an appropriate medium, a cylindrical channel analogous to a conventional step-index fiber may be induced.
In U.S. Pat. No. 4,585,301, a transverse radiation profile approximating a prism is created in a absorptive medium by use of a carbon dioxide laser emitting a beam having a wavelength of 10.6 microns at an energy level of 0.1 joules for a duration of 1.7 nanoseconds. Under these conditions, a thermal gradient in the shape of a prism, or a thermal lens is created in the absorptive medium due to thermal changes created by the optical energy. A signal beam is inputted into the medium parallel to the control beam and is angularly deflected as it passes through the thermal lens. By varying the energy of the control beam, the slope of the thermal gradient or lens is correspondingly varied to cause either greater or lesser deflection as desired.
It is important to note that in the optical prism device described in the above-referenced U.S. application, while the signal beam is angularly deflected after passing through the medium, it undergoes no change in size or area. That is, if it enters the medium as a small beam, it will emerge therefrom as a single small beam. Because of this one-to-one size relationship between incident beam and deflected beam, the prism device disclosed in this patent is not suitable for some applications. For example, in many instances it is desirable to have a single incident beam control or "turn on" a number of secondary switches in a cascading or series effect. The small, circular beam deflected by the prism cannot be readily employed in this manner since it is difficult to split up. Furthermore, in order to create the proper thermal gradient necessary to give rise to the thermal lens or prism, a relatively long duration of time for the switch beam to act on the absorptive medium is necessary, i.e., on the order of 170 nanoseconds. Hence, the switching time of this prism device is considerably slower than that of a conventional electronic switch, whose switching time may be on the order of nanoseconds.
It would be desirable to provide a means of thermo-optically creating and controlling a waveguide in a partially absorptive medium whereby an optical beam could be guided through said medium, in a manner analogous to present fiber optic devices.
It would be highly desirable to optically guide such an optical beam through such a medium such that the guided beam emerges as a diverging, cone-shaped beam, which way be split into a number of secondary beams to control multiple switches simultaneously.
It would also be highly desirable for the operating speed of such a thermo-optical waveguide to be as good as or better than conventional electronic switches. A device of this type would have significant utility in the fabrication of optical switching systems such as optical computers, communication networks and a variety of electro-optical devices.